The present invention relates to photographic materials containing a new class of electron-accepting compounds.
Electron accepting agents are useful for both negative and direct positive working silver halide photographic materials. For negative working materials electron-accepting agents are usually called spectral desensitizers. They are able to lower the sensitivity of the material in the spectral range of their absorption. Of course, combinations of desensitizers with different spectral regions are possible.
Prior art concerning electron-accepting compounds suitable for use in negative working or direct positive emulsions, includes nitrostyryl and nitrobenzylidene dyes as described in U.S. Pat. No. 3,615,610, dihydropyrimidine compounds of the type disclosed in DE 2,237,036 and compounds of the type disclosed in U.S. Pat. No. 3,531,290. Other useful electron accepting compounds are cyanine and merocyanine dyes containing at least one nucleus, and preferably two nuclei with desensitizing substituents such as nitro groups, or dyes containing desensitizing basic nuclei as described in U.S. Pat. Nos. 2,930,644, 3,431,111, 3,492,123, 3,501,310, 3,501,311, 3,574,629, 3,579,345, 3,598,595, 3,592,653, and GB 1,192,384.
Electron accepting compounds belonging to the class of nitrophenylthioether derivatives are disclosed in U.S. Pat. No. 4,820,625.
In the sector of pre-press activity known as graphic and reprographic arts an intensive use is made of contact copying materials to reproduce screen dot images, line work and typesetting work. Both negative working photographic materials which produce negative-positive or positive-negative copies are used as well as so-called direct positive working materials giving rise to negative-negative or positive-positive reproductions.
In order to obtain exact copies with sharp dot and line edges, it is necessary to use fine-grained relatively insensitive photographic emulsions. The materials containing this type of emulsions are image-wise exposed in contact with the original in a graphic arts copying apparatus by means of high intensity radiation, preferably by light sources emitting a high content of near-ultraviolet light. Common light sources for this purpose are mercury vapour lamps, metal-halogen lamps, xenon tubes, pulsed xenon tubes and quartz-halogen sources.
The handling of ever increasing amounts of photographic materials of different kinds, the decentralisation of the distinct steps in the reproduction cycle etc, have created a demand for silver halide materials which can be handled under clear ambient light illumination. This demand has given rise to the development of so called "roomlight materials" which can be image-wise exposed, handled and processed without sensitometric changes in a reasonable time interval while being illuminated by common office fluorescent tubes and daylight penetrating through office windows. Prior art material which can be handled under roomlight conditions has been described in e.g. U.S. Pat. No. 2,219,667 and GB 1,330,044.
Silver halide emulsions contained in such roomlight materials should exhibit adequate sensitivity and other sensitometric characteristics for image-wise exposure while showing no photographic response under ambient light conditions. It is the task of the emulsion designer to establish the optimal compromise between these two conflicting characteristics.
For negative working roomlight applications the intrinsic sensitivity of silver halide is usually too high; lowering of the sensitivity is possible by incorporating internal electron traps being centers promoting the deposition of photolytic silver in the interior parts of the silver halide grains and/or by the addition of electron-accepting agents to the silver emulsion, as is the case in this application. The latter method shows the advantage that these spectral desensitizers usually induce a pronounced Low Intensity Reciprocity Failure (LIRF) which is one of the factors promoting excellent roomlight stability. This excellent roomlight stability further requires the use of emulsions mainly composed of chloride (at least 70%) the spectral sensitivity distribution of which is restricted to the near ultraviolet. Incorporation of too much bromide or iodide in the emulsion extends the spectral sensitivity distribution too much into the visible region (see F. Moser and R. K. Ahrenkiel in "The Theory of the Photographic Process", edited by Th. James and published by Macmillan Publ. Co., Inc., New York (1977), p. 39.). A spectral desensitizer when present should show a similar spectral absorption limited to the near ultraviolet in order to avoid extension of the sensitivity into the visible region.
Direct positive working roomlight emulsions can function according to different emulsion technology principles. They can contain internal traps due to the presence of phase boundaries in the so-called core-shell emulsion type. They can function according to internal electron trapping due to the building in of inorganic desensitizers, e.g. metal dopants, in the interior of the silver halide crystals. In these cases of internal trapping, the emulsion surface can be fogged or not; in the latter case usually a reducing agent is present in the photographic material or in its developing solution, e.g. a hydrazine derivative. Finally they can work according to the principles of external electron trapping in which case the emulsion surface is prefogged and an electron-accepting compound is adsorbed at it.
Relatively sensitive direct positive emulsions can be composed of AgBr or AgBrI; in this case however red safety light conditions as present in classical darkrooms are required. Rather insensitive direct positive AgBr(I) emulsions exist, suited for exposure by quartz-halogen sources, which can be handled under relative bright yellow light conditions. For roomlight direct positive applications however it is, like in the corresponding negative case, necessary to use emulsions with a high content of chloride in order to minimize the overlap between the intrinsic sensitivity and the roomlight spectrum. If the absorption of the external electron-accepting agent would extend to the visible region, as it is e.g. the case with the well known electron-accepting agent Pinakryptol Yellow (further indicated as Reference Compound R-1), this would lead under roomlight to the bleaching of the latent image created by the prefogging of the emulsion.
It is one of the purposes of the present invention to provide a new class of electron acceptors which do not show the disadvantage of inadequate spectral absorption.